'\" et
.TH TIMER_GETOVERRUN "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
timer_getoverrun,
timer_gettime,
timer_settime
\(em per-process timers
.SH SYNOPSIS
.LP
.nf
#include <time.h>
.P
int timer_getoverrun(timer_t \fItimerid\fP);
int timer_gettime(timer_t \fItimerid\fP, struct itimerspec *\fIvalue\fP);
int timer_settime(timer_t \fItimerid\fP, int \fIflags\fP,
    const struct itimerspec *restrict \fIvalue\fP,
    struct itimerspec *restrict \fIovalue\fP);
.fi
.SH DESCRIPTION
The
\fItimer_gettime\fR()
function shall store the amount of time until the specified timer,
.IR timerid ,
expires and the reload value of the timer into the space pointed to by
the
.IR value
argument. The
.IR it_value
member of this structure shall contain the amount of time before the timer
expires, or zero if the timer is disarmed. This value is returned as
the interval until timer expiration, even if the timer was armed with
absolute time. The
.IR it_interval
member of
.IR value
shall contain the reload value last set by
\fItimer_settime\fR().
.P
The
\fItimer_settime\fR()
function shall set the time until the next expiration of the timer
specified by
.IR timerid
from the
.IR it_value
member of the
.IR value
argument and arm the timer if the
.IR it_value
member of
.IR value
is non-zero. If the specified timer was already armed when
\fItimer_settime\fR()
is called, this call shall reset the time until next expiration to the
.IR value
specified. If the
.IR it_value
member of
.IR value
is zero, the timer shall be disarmed. The effect of disarming or
resetting a timer with pending expiration notifications is unspecified.
.P
If the flag TIMER_ABSTIME is not set in the argument
.IR flags ,
\fItimer_settime\fR()
shall behave as if the time until next expiration is set to be equal
to the interval specified by the
.IR it_value
member of
.IR value .
That is, the timer shall expire in
.IR it_value
nanoseconds from when the call is made. If the flag TIMER_ABSTIME is
set in the argument
.IR flags ,
\fItimer_settime\fR()
shall behave as if the time until next expiration is set to be equal
to the difference between the absolute time specified by the
.IR it_value
member of
.IR value
and the current value of the clock associated with
.IR timerid .
That is, the timer shall expire when the clock reaches the value
specified by the
.IR it_value
member of
.IR value .
If the specified time has already passed, the function shall succeed
and the expiration notification shall be made.
.P
The reload value of the timer shall be set to the value specified by the
.IR it_interval
member of
.IR value .
When a timer is armed with a non-zero
.IR it_interval ,
a periodic (or repetitive) timer is specified.
.P
Time values that are between two consecutive non-negative integer
multiples of the resolution of the specified timer shall be rounded up
to the larger multiple of the resolution. Quantization error shall not
cause the timer to expire earlier than the rounded time value.
.P
If the argument
.IR ovalue
is not NULL, the
\fItimer_settime\fR()
function shall store, in the location referenced by
.IR ovalue ,
a value representing the previous amount of time before the timer would
have expired, or zero if the timer was disarmed, together with the
previous timer reload value. Timers shall not expire before their
scheduled time.
.P
Only a single signal shall be queued to the process for a given timer
at any point in time. When a timer for which a signal is still pending
expires, no signal shall be queued, and a timer overrun shall occur.
When a timer expiration signal is delivered to or accepted by a
process, the
\fItimer_getoverrun\fR()
function shall return the timer expiration overrun count for the
specified timer. The overrun count returned contains the number of
extra timer expirations that occurred between the time the signal was
generated (queued) and when it was delivered or accepted, up to but not
including an implementation-defined maximum of
{DELAYTIMER_MAX}.
If the number of such extra expirations is greater than or equal to
{DELAYTIMER_MAX},
then the overrun count shall be set to
{DELAYTIMER_MAX}.
The value returned by
\fItimer_getoverrun\fR()
shall apply to the most recent expiration signal delivery or acceptance
for the timer. If no expiration signal has been delivered for the timer,
the return value of
\fItimer_getoverrun\fR()
is unspecified.
.P
The behavior is undefined if the value specified by the
.IR timerid
argument to
\fItimer_getoverrun\fR(),
\fItimer_gettime\fR(),
or
\fItimer_settime\fR()
does not correspond to a timer ID returned by
\fItimer_create\fR()
but not yet deleted by
\fItimer_delete\fR().
.SH "RETURN VALUE"
If the
\fItimer_getoverrun\fR()
function succeeds, it shall return the timer expiration overrun count
as explained above.
.P
If the
\fItimer_gettime\fR()
or
\fItimer_settime\fR()
functions succeed, a value of 0 shall be returned.
.P
If an error occurs for any of these functions, the value \-1 shall be
returned, and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fItimer_settime\fR()
function shall fail if:
.TP
.BR EINVAL
A
.IR value
structure specified a nanosecond value less than zero or greater than
or equal to 1\|000 million, and the
.IR it_value
member of that structure did not specify zero seconds and nanoseconds.
.P
The
\fItimer_settime\fR()
function may fail if:
.TP
.BR EINVAL
The
.IR it_interval
member of
.IR value
is not zero and the timer was created with notification by creation of
a new thread (\c
.IR sigev_sigev_notify
was SIGEV_THREAD) and a fixed stack address has been set in the thread
attribute pointed to by
.IR sigev_notify_attributes .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
Using fixed stack addresses is problematic when timer expiration is
signaled by the creation of a new thread. Since it cannot be assumed
that the thread created for one expiration is finished before the next
expiration of the timer, it could happen that two threads use the same
memory as a stack at the same time. This is invalid and produces
undefined results.
.SH RATIONALE
Practical clocks tick at a finite rate, with rates of 100 hertz and
1\|000 hertz being common. The inverse of this tick rate is the clock
resolution, also called the clock granularity, which in either case is
expressed as a time duration, being 10 milliseconds and 1 millisecond
respectively for these common rates. The granularity of practical
clocks implies that if one reads a given clock twice in rapid
succession, one may get the same time value twice; and that timers must
wait for the next clock tick after the theoretical expiration time, to
ensure that a timer never returns too soon. Note also that the
granularity of the clock may be significantly coarser than the
resolution of the data format used to set and get time and interval
values. Also note that some implementations may choose to adjust time
and/or interval values to exactly match the ticks of the underlying
clock.
.P
This volume of POSIX.1\(hy2017 defines functions that allow an application to determine
the implementation-supported resolution for the clocks and requires an
implementation to document the resolution supported for timers and
\fInanosleep\fR()
if they differ from the supported clock resolution. This is more of a
procurement issue than a runtime application issue.
.P
If an implementation detects that the value specified by the
.IR timerid
argument to
\fItimer_getoverrun\fR(),
\fItimer_gettime\fR(),
or
\fItimer_settime\fR()
does not correspond to a timer ID returned by
\fItimer_create\fR()
but not yet deleted by
\fItimer_delete\fR(),
it is recommended that the function should fail and report an
.BR [EINVAL] 
error.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIclock_getres\fR\^(\|)",
.IR "\fItimer_create\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<time.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
